A previous patent to the inventor herein and Theodore W. Sippel is U.S. Pat. No. 4,570,071, issued Feb. 11, 1986, and is hereby incorporated by reference.
FIG. 1 illustrates a detector array 3. An X-ray source 6 produces an X-ray beam 9 which passes around and through an object 12 under scrutiny and then enters a chamber (not shown) containing the detector array 3. The chamber is filled with pressurized Xenon gas in the presence of an electric field indicated by arrow 15. The field spans between a charged plate 18 and the detector array 3. The X-rays ionize the Xenon and the electric field drives the electrons resulting from ionization toward individual detector elements 3A-K. The electrons build up a charge on the detectors 3A-K which is a function of the amount of ionization, which is, in turn, a function of the intensity of the incoming x-radiation.
Restated, the charge distributed among the individual detector elements 3A-K indicates the spatial intensity distribution of the incoming x-radiation. Consequently, the charge distribution can be used to construct an image of the object.
The intensity of radiation produced by the X-ray source 6 tends to fluctuate unpredicably over time. Thus, knowledge of the intensity of x-radiation striking the detector array 3 (derived from the charge distribution) is, by itself, insufficient to indicate the X-ray attenuation caused by the object 12: the intensity at the source 6 is not known.
One solution to this problem is to provide additional, reference, detector elements 21A-G at about the same distance from the X-ray source 3 as detectors 3A-K (now called data detectors), but spaced by space 24 such that the reference detectors 21A-G always maintain an unobstructed line of sight to the X-ray source. That is, the object 12 never obstructs the X-rays received by the reference detectors 21A-G. Comparison of the X-ray intensity indicated by the data detectors with that indicated by the reference detectors allows one to infer the attenuation caused by the object 12.
Two problems arise with the space 24 in the detector arrangement shown. The first problem relates to safety. The electric field 15 above the detector elements 31A-K is of the order of 850 volts per 35 milli-inches (distance 27 is about 0.035 inches), or roughly 24,000 volts per inch. Consequently, the electrons produced by ionization will be driven toward space 24, where they will collect. In theory, the electrons will collect until space 24 acquires the same potential as plate 18, namely, 850 volts. However, in practice, dielectric breakdown will probably occur first: The electron accumulation in space 24 will arc over to a neighboring detector element such as 3K, causing damage to the detector array 3 and to associated electronic equipment (not shown).
A second problem is that, under the configuration shown, the electric field will probably resemble the undesirable curved, branched configuration indicated by arrows 30. It is preferred that the electric field lines be straight and perpendicular to the data detector elements 3A-K, as is field-arrow 15. When perpendicular, the fields lines cause the electrons from ionization to travel directly downward toward detectors beneath them, and not toward a detector on one side or the other. This direct travel is necessary for reasons relating to the reconstruction of an image of the object 12 in FIG. 1, and need not be understood here.